Really depends on how large the effects used will be in-game. Most explosion effects are fine as 512^2, but shockwaves which can be larger than even destroyers should be 1024^2. If you clearly differentiate large explosions from small ones, you can use both 256^2 and 512^2 effects, but if you mix them up, then going with just 512^2 is probably better. For example, flak and missile explosions may not necessarily be restricted to flaks and missiles, but if they are 256^2, they probably end up being ugly due to their low resolution to be used elsewhere.
But as I thought this was about CREATION of said effects, then going with at least twice the intended in-game resolution is safe bet. If your PC is powerful enough to keep render times reasonable, then going with four times intended in-game resolution would be great. That avoids the trouble of having to re-render effects again because you suddenly realize there's need for higher res version of some effect.
For example, Nighteyes did not originally render his shockwaves high resolution enough, making their quality worse compared of the current mediavps ones. Same can be said of the red flash effect. A 1024^2 effect and a small 256^2 effect would probably be most optimal setup, the bigger for capitals and smaller for bombs. But as it is now, 512^2 is used for both.
Also FYI, 1024^2 DXT1 works out better than 512^2 uncompressed if and when you're aiming for quality. Of course, same applies in other resolutions as well.
